1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same, particularly, to an improvement in the spacer used in the liquid crystal display device.
2. Description of the Related Art
In a liquid crystal display device, the display characteristics of the device are seriously affected by the distance between a pair of mutually opposing substrates included in the device. Specifically, if the distance between the substrates is not uniform over the entire region of the display panel, the display quality of the device is deteriorated. For example, the color and the display on the display panel are rendered uneven. Also, interference stripes appear on the display panel. In recent years, the liquid crystal display panel is required to be highly accurate and tends to become larger and larger in size, with the result that it is more important nowadays to increase the uniformity in the distance between the substrates in order to obtain a liquid crystal display panel of a higher accuracy and a larger display area than in the past.
A liquid crystal display device, which is intended to meet the requirements noted above, is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 1-134336. The device disclosed in this prior art comprises a first substrate, columnar spacers, and a second substrate. Pixel electrodes, a wiring, etc. are arranged in a matrix form on the surface of the first substrate. An alignment film made of a polyimide is formed on the first electrode to cover these electrodes, etc. followed by forming the columnar spacers made of a light-sensitive polyimide on the alignment film. These columnar spacers are formed by, for example, photolithography. Further, the second substrate is disposed to face the first substrate with the columnar spacers interposed therebetween. Since photolithography is employed, the columnar spacers can be formed in a desired shape at desired positions. Such being the situation, vigorous studies are being made nowadays on the columnar spacer in accordance with increase in the size of the display panel.
In general, the surface of the alignment film made of a polyimide is rubbed with cloth such as velvet to cause an initial orientation of the liquid crystal in a predetermined direction. Where columnar spacers are formed on the alignment film as in the prior art exemplified above, the rubbing treatment of the alignment film formed on the substrate is followed by a process of forming the columnar spacers. To be more specific, the oriented polyimide film is coated with a light-sensitive polyimide, followed by exposure to light and development (etching) so as to form the columnar spacers of a desired shape at desired positions. However, the etching solution causes the surface region of the polyimide alignment film positioned below the light-sensitive polyimide film to be swollen and dissolved in the etching solution. It follows that the formation of the columnar spacers markedly lowers the effect of the rubbing treatment to orient the polyimide alignment film. As a result, when a liquid crystal is loaded in the free space formed between the two substrates included in the liquid crystal display device comprising columnar spacers, the liquid crystal molecules are oriented uneven, leading to deterioration in the quality and contrast of the picture image displayed on the display panel. Further, the on-off control of light cannot be achieved.
To overcome the difficulties described above, it is proposed to apply the rubbing treatment after formation of the columnar spacers. In this method, however, fur of the rubbing cloth is caught by the columnar spacer, with the result that the polyimide film is not sufficiently oriented in the region near the spacer. It follows that a region of defective orientation is brought about around the columnar spacer, giving rise to a marked deterioration in the displayed picture image quality. Proposed in Japanese Patent Disclosure No. 3-127028 is an idea of using a columnar spacer which tapers upwardly such that a vertical cross section of the spacer forms a trapezoid. It is proposed that the angle formed between the alignment film surface and the side surface of the tapering spacer, which is naturally greater than 90.degree., should be about 135.degree. so as to allow the fur of the rubbing cloth to be unlikely to be caught by the spacer. Even in the case of using the tapering columnar spacer, however, it is impossible to prevent completely the fur from being caught by the spacer in the rubbing step, resulting in failure to improve sufficiently the displayed picture image quality.
It should also be noted that, in general, the liquid crystal around the spacer is affected by the spacer surface, giving rise to a region of disturbed crystal orientation. FIG. 1 schematically shows how the crystal orientation is disturbed around the spacer included in a liquid crystal cell. As shown in FIG. 1, a region 144 of a disturbed orientation is formed around a spacer 112. The disturbance of orientation is gradually increased inward from alignment films 142, which serve to control the liquid crystal orientation, of the upper and lower substrates, reaching the greatest disturbance in the central region of the cell. Naturally, the disturbed region is much widened in the cell central region. Since the liquid crystal molecules in the disturbed region 144 are incapable of achieving an optical rotation, the light cannot be turned ON or OFF in accordance with ON or OFF of the voltage application. In other words, when voltage is not applied across the cell, the incident light is not rotated in the region of the spacer 112 and the region 144 around the spacer in which the crystal orientation is disturbed. Thus, if a white level is displayed in the normally white mode, black dots are observed in the region of the spacer 112 and the region 144 around the spacer, leading to deterioration in the display quality. Likewise, if a black level is displayed in the normally black mode, white dots (light leakage) are observed in the region of the spacer 112 and the region 144 around the spacer, which also leads to deterioration in the display quality.
In the conventional liquid crystal display device, the disturbance of the crystal orientation is much widened around the columnar spacer, particularly in the central region of the cell, with the result that the area of the black dot or white dot (light leakage) appearing on the display screen is rendered much larger than the area of the spacer 112 so as to be sufficiently recognized by the naked eye.